1. Field of the Invention
This invention relates to a strained superlattice light emitting device and, more particularly, it relates to a superluminescent diode having a wavelength band around 0.8.mu.m with a controlled plane of polarization.
Prior Art
Superluminescent diodes (hereinafter referred to as SLDs) are classified between semiconductor laser devices and light emitting diodes and have been attracting attention in recent years. Such a light emitting device shows an incoherent and wide light emitting spectrum like a light emitting diode and, at the same time, can emit highly directional light with an enhanced output level like a semiconductor laser device.
In order to use an SLD, it is necessary to effectively control the polarization dependency of the output light of the device.
There has been discussed the application of a technique developed for strained superlattice structures to the control of polarization related properties of SLDs having a wide wavelength band between 1.3 and 1.55.mu.m. The technique is based on the phenomenon that, when a superlattice structure is subjected to bidirectional strain, it comes to show a modified vallence band structure. The gain for the TE mode which relates to heavy holes increases relatively, if the applied strain is compressive, whereas the gain for the TM mode which relates to increases relatively light holes, if the strain is tensile.
Meanwhile, known techniques for manufacturing SLDs and superlattice optical amplifiers capable of producing polarization independent light include the following:
1) The use of a multiple quantum well structure for the active layer, strain free GaInAs for the well layer and GaInAs with in-plane tensile strain for the barrier layer in an SLD (See Paper No. 1 listed below). PA1 2) Generation of an appropriate degree of tensile strain in the well layer of the multiple quantum well structure of an optical amplifier (See Paper No. 2 listed below). PA1 Paper No. 1: IEEE J: Quantum Electron., vol.127, pp. 1463, 1991. PA1 Paper No. 2: Optical Fiber Topical Meeting FA3, 1992. PA1 1) If the first one of the above listed techniques is used for a 0.85 .mu.m band on a GaAs substrate of a device of the type under consideration, the wavelength of light emitted from the strain free GaAs well layer is shifted to become shorter by the quantum confining effect of the device, which consequently can no longer emit light with a same wavelength of 0.85 .mu.m. This phenomenon is particularly disadvantageous because it can alter the branching effect of the fiber coupler. PA1 2) If the density of injected current or that of injected carriers goes out of a predetermined range with the second technique, heavy and light holes come to show a change in the distribution pattern. This in turn produces a lopsided relationship in the intensity of light emitted in the TE mode and in the TM mode to raise the polarization dependency of the output.
A proposed theory explaining the mechanism of achieving polarization independency by this technique is as follows:
A barrier layer that is tensile strained by approximately 2% shows a valence band energy level for holes lower than that of a strain free well layer. This by turn increases the concentration of holes within the barrier layer. Since these holes are mostly light holes, the rate at which electrons and light holes are recombined to emit light in the TM mode is increased. On the other hand, since holes within the well layer behave as heavy holes, they are mostly active for emitting light in the TE mode. This well balanced activities of holes in the emission of light provides the basis for achieving polarization independency of the output of the SLD.
According to a proposed theory explaining the mechanism of achieving polarization independency by this technique, holes in a strain free well layer are normally heavy holes that are mostly active for emitting light in the TE mode. As the well layer is tensile strained, the energy level of light holes is lowered to increase the concentration of holes within the light hole band. Since light holes are mostly active for the emission of light in the TM mode, the intensity of light emitted in the TE mode and that of light emitted in the TM mode can be balanced to realize polarization independency of the output of the optical amplifier.
[Problems to be Solved by the Invention]
However, known techniques including the above are accompanied by the following problems if they are used to achieve polarization independency of the output of devices of the type under consideration.
In view of the above identified problems, it is therefore an object of the present invention to provide a strained superlattice light emitting device which is of polarization independency, regardless of the density of injected current or that of injected carriers or which shows an intensity of light emitted either in the TE or TM mode by far greater than that of light emitted form an ordinary bulk active layer.